1. Field of the Invention
This invention relates to a foundation (and a corresponding method of foundation installation) for anchoring and supporting a tower, such as transmission line towers and the like, with the foundation hereof being easily and inexpensively installed to provide a high-strength foundation for such a tower. More particularly, it is concerned with a screw anchor foundation which can accommodate slight anchor installation errors by provision of a connector-receiving cavity adjacent the upper end of the anchor which receives an initially flowable, solidifiable fill material (such as synthetic resin) along with an elongated tower leg connector; the latter can be accurately positioned relative to the tower leg to present, upon setting of the fill material, a properly aligned, rigid tower leg connection.
2. Description of the Prior Art
As those skilled in the art will appreciate, large upright power transmission line towers can present difficult problems from the standpoint of providing proper foundation support. Specifically, a great deal of weight must be supported, the tower must be positioned to avoid undue bending or torsional loads, and the tower foundation must be able to withstand extreme, wind-induced loads imposed on the tower. Heretofore, the most prevalent method of anchoring free standing transmission line towers has been the use of concrete pads poured in place at the four corners of the tower for supporting the tower legs. As can be appreciated, the concrete pads must be sufficiently embedded in the earth and reinforced to withstand the tension, compression and wind loading imposed on the overall structure. Typically, in such an installation an adjustable leg anchor is embedded in the concrete prior to setup, so that upon hardening of the cement, the leg anchors can be used to connect the respective tower legs to the concrete pad.
Although the use of properly constructed concrete pads to anchor transmission line towers is a practice of long standing, many problems are presented with such an anchoring method. For example, the size and strength of the concrete pad required for the particular application will vary according to the tower size, soil conditions, conductor stringing loads and anticipated wind loads. Additionally, constructing such concrete pads requires hauling to the job site excavation equipment, concrete forms, and often a large amount of concrete, all of which can be difficult in remote regions far from adequate access roads. Further, in some locations, such as swampy ground, it is difficult and very expensive to construct proper concrete pads.
These difficulties have led to suggestions of using conventional screw anchors as foundations for transmission line towers. Screw anchors have the advantages of easy installation, relatively easy transportation to the job site, and adaptability for use in many types of soil conditions, including swampy ground. However, the straightforward approach of simply installing four screw anchors into the earth and simply attaching tower legs thereto has proven to be impractical. This stems from the inherent difficulty of driving an anchor into the ground with the required precision that the upper extremity of such anchor is exactly aligned with a respective tower leg for direct bolting of the latter to the anchor shaft or tube. Not only is it impractical to install the screw anchor at an exact required angle with respect to the vertical because of the inability to start such anchor with required accuracy, but equally as important, differences in the stratum of the ground as the anchor is driven into the latter causes deflections of the anchor to one side or the other depending upon the nature of the soil. As a result the upper end of the anchor after completion of the installation therof is not necessarily in the precise position required for alignment with a respective tower leg.
It has been suggested in this regard that perhaps the screw anchor foundations can be welded to the tower legs and if necessary intermediate metal shims or connectors may be welded between the screw anchor foundation and the corresponding tower legs to compensate for misalignment of the anchor with the tower leg extension. Although field welding might in many instances solve the problem presented, it is not a satisfactory solution from a construction standpoint because of the difficulty of controlling the quality of the welds under field conditions, the need to transport the necessary welding apparatus to the construction site, the absolute requisite of employing highly skilled welders who can perform quality welding under extremely adverse field conditions, and the reluctance of utilities to support very high volatage electrical conductors on towers where the integrity of the system is dependent on the quality of the welding which has not and cannot be subjected to the same analytical techniques available in shop welding fabrication.
A recent breakthrough in the art is described in U.S. Pat. No. 4,339,889. This patent describes an adjustable connector for tower legs which overcomes the problems outlined above and greatly facilitates use of screw anchors as foundations for upright towers. This device avoids the need for field welding, but is still relatively expensive and requires considerable effort to install.